In Chapterhouse Dune and in Hunters of Dune, it is clear that the Sandworms are causing the desert area to grow and take more and more of the surface of the planet. However, the sandworms need desert-sand to be able to move (as implied by their name). Do they actually turn other types of ground into sand? Is this something caused by the melange or does it happen in some other way?
It's part of the life-cycle of the sandworms. In their initial stage, they are small sandtrout. These sandtrout work together to isolate any water in the environment, which they then mix with. This results in a subsurface pre-spice mass. Eventually, this pre-spice mass builds enough pressure to erupt out of the ground as a spice blow. Any sandtrout that survive the eruption then combine to form small, young sandworms.
Due to this, any environment with sandtrout present will eventually become desert as the moisture is removed from the ground and thus the environment at large. Precipitation will enter the ground, then be converted to the pre-spice mass, interrupting the water cycle and preventing water from evaporating and re-entering the atmosphere to precipitate again. This suppresses the amount of precipitation, leading to desert conditions. As the number of sandworms and sandtrout increase, the desert will spread.
More information can be found at the Dune wikia.
The sandtrout, which is the larval stage of the worms, encase any source of water or moisture they can find as part of the conversion to pre-spice mass. The process of walling off moisture removes it from the ecosystem, resulting in desert conditions.
The more sandworms there are, the more sandtrout. The more sandtrout, the less moisture is in the ecosystem, which results in more arid (desert) regions.
In addition to the climactic impact, it seems likely that the abrasion of worms moving through ground would contribute to grinding rock and dirt into finer particles, resulting in faster development of sandy desert conditions.
The factors involved in the ongoing desertification:
- Sandtrout wall off moist areas in the sand
- if the water is in a dry enough environment, the convert it to the pre-spice mass
- if the water is is not in a dry enough environment, or is a large enough body, it merely builds up a layer of dead sandtrout, creating a barrier to that water migrating nearby and thus preventing it's use by plants. (Read the descriptions of Jacrutu.)
- Sandtrout isolate and encapsulate root systems on the plants on the sand's edge.
- this process would be similar to the process Leto II experiences.
- encapsulated roots can draw nutrient only from the sandtrout; this will kill most trees and plants by starvation.
- as plants starve off and die, the edge of the desert expands, as their decomposition allows the sandtrout to push further back in the root mass.
- as the edge dries, sandtrout are also able to form the pre-spice mass, and when that mass generates a spice blow, it liberates the soil, and blows sand over the desert edge, also hastening the edge migration.
- Desert erosion of plant free surfaces increases sand.
- the more sand, the more water migrates under the sand.
- due to the sandtrout, moist sections that are deep enough in the sand will be capped off.
- likewise, water that hasn't migrated deep will be converted to spice.
- the sandworm friction results in heating
- this increases the vapor release from the sand to the atmosphere
- this converts a portion of the biological energy into heat, which is radiated from the ground.
- increased ground heat results in harsher climate at basin edge, with more severe wind, and nutrient stripping into the desert low-pressure system, as wel as moisture dump at the edges of the desert, where sand trout can encapsulate and isolate pools.
- Sandworm movement results in friction-erosion of basin bottoms and solid objects in the sand.
- this creates more and deeper sand
- this reduces dry biological particle sizes
- this reduces rocks and spires in the sand basins to smaller/thinner ones.
- sandworms eat biological material mixed in sand; this includes sand-top photosynthetic plantonids, as well as plant debris blown from desert edge.
- This reduces the amount of nutrient available in the sand for plants to absorb, preventing dune fixation by fast but shallow cover plants.
- this also limits moisture release by decaying biologicals.
- the sandworm friction results in heating
When climate becomes very dry, most of the vegetation dies out. As a result, the soil will dry up and get blown away by wind as grains of sand, without plants to fix it with their roots. Forest-covered hills or mountains will be stripped to the bare rock. Then more sand will be created by wind erosion: particles carried by wind would grind against rock, making more particles.
In short, if you remove moisture from an ecosystem, you will get a lot of sand. That's what happened to the Sahara and to Australia.